1. Field of the Invention
The invention relates to structural trusses and in particular structural trusses in which the camber of the load bearing upper chord member may be selectively adjusted.
2. Description of the Related Art
Trusses are common structures for supporting a load. They are used most often to support roofs, bridges and other loads spanning relatively large distances.
For improved efficiency in bearing flexural and axially loads the trusses have been designed to permit prestressing the entire truss. Members of the truss that would be in tension under load are placed in compression by prestressing so that when the working load is applied the stresses are reduced by the amount of the pre-stress. Because each member of such a truss is subjected to compression, the most damaging stress, either as a part of the pre-stress or under application of the working load, each member of the truss must be designed and sized for the most damaging compressive stress. Moreover, in conventional trusses, the working member is supported by a structure which also resists efforts to adjust the camber of the working member; any effort to adjust the camber of a working member would require deflection of the entire stiffness of the truss.
In the subject invention, by contrast, the working member, the top chord member, may be slightly pre-stressed by camber adjustment and is the only member, with the exception of the king posts, subject to compression. The other members forming the truss act in unison to resist a load acting on the working member, but act independently when adjusting the camber of the working member. The other members, therefore, only experience slight increases in stress during cambering of the working member. Moreover, the stress is tensile, not compression. The structure of the invention permits camber adjustment of the upper chord member against the stiffness of just the upper chord member since the other supporting members do not resist camber adjustment. Because only the upper chord member and king post are subjected to compression, the remaining members do not have to be sized to accommodate such forces. These benefits and others are achieved through use of the invention as described in the specification as follows. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations as particularly pointed out in the appended claims.